Granular filtration medium is occasionally used in the treatment of raw water (e.g., sewage, industrial effluents) for removing oily matter and insoluble solids suspended in the raw water. During the filtration process residual suspended material is filtered out and retained in the filter bed, and after a period of use the granulated medium becomes clogged due to formation of mud balls and solidifications caused by the filtration residuals.
For example in the sand and rubble stone depth filtration apparatus of AMIAD Filtration SYSTEMS (www.amiad.co.il/filters/sandMediaFilters_3.asp) the filtrate is passed from the filtration medium via an array of nozzles provided in the base of the vessel comprising the filtration medium. In this apparatus the filtration medium is typically comprised of two layers; a first layer of rubble stone which covers the array of nozzles, and a second layer of sand which covers the rubble stone layer, wherein the main purpose of the rubble stone layer is to prevent the fouling of the nozzles by sand particles. This array of nozzles is susceptible to fouling and thus requires frequent treatment or replacement.
Typically, a backwash or pressure wash process is applied in order to recover the clogged filtration medium and release the fouling of the nozzles. In order to effectively clean the granular filtration medium from the retained filtration residuals continuous backwash treatments are required, which consumes great amounts of fresh water. Furthermore, due to the frequent backwashes typically needed to open the clogged nozzles channels are formed over time in the filtration medium which significantly reduce the efficiency of the filtration apparatus due to the tendency of the raw water introduced into the device to pass through the formed channels directly to the nozzles i.e., without passing through the sand grains of the filtration medium.
A washer apparatus is described in JP 8215509 for washing and regenerating a clogged filter medium in a moving filter bed type filter using a granular filter medium such as sand, wherein contaminated filter medium is washed while rising in a screw conveyor vertically installed in the filter medium bed, in which the filter medium is crushed and then washed by an agitator.
JP 8266815 describes a device in which a filter medium is washed and circulated by means of a lift pipe vertically installed in the central part of the filter tank for conveying the filter medium.
In the device described in U.S. Pat. No. 4,102,786 clogged filter medium is treated by introducing upward current of water from the lower side of the filter bed and circulating filtering grains via a circulation line back into the device via the lower side of the filter bed, thereafter a current of rinsing water is introduced from the lower side of the bed to wash out filtration residuals.
The methods described above have not yet provided satisfactory solutions for rapidly and efficiently refreshing and restoring a clogged granular filtration medium with a relatively small amount of water.
It is therefore an object of the present invention to provide a filtration apparatus and method and apparatus for efficiently operating a granular medium filtration apparatus and for refreshing and regenerating clogged filter medium.
It is another object of the present invention to provide a method and apparatus for granulating a clogged filter medium, separating mud balls and other solidifications, and for washing out filtration residuals therefrom.
It is a further object of the present invention to provide a filtration apparatus employing a granular filtration medium which is less susceptible to blockage and which is easy and simple for maintenance.
Other objects and advantages of the invention will become apparent as the description proceeds.